1. Field of the Invention
The present invention relates to a high frequency switch or the like, of which main object is to switch a high frequency signal in a radio circuit of radio communication equipment such as a portable telephone.
2. Description of the Prior Art
High frequency switch circuits are often used to switch transmit/receive signals in radio circuits of radio communication equipment such as a portable telephone using TDMA systems.
Hereinafter, an example of conventional high frequency switch circuits described above will be described with reference to a drawing.
FIG. 13 shows an equivalent circuit diagram of an example of conventional high frequency switch circuits.
In FIG. 13, the anode of a first diode D1301 is coupled to a transmitting terminal 1301 through a first capacitor element C1301. Further, a control terminal 1302 is coupled to the anode side of the first diode D1301 through an inductor element L1301 and a resistor element R1301. Also, the anode of a second diode D1302 is coupled to a receiving terminal 1303 through a second capacitor element C1302, and the cathode of the second diode is connected to ground. Further, one end of a first transmission line TL1301, which has an electrical length of ¼ wavelength at the operating frequency, is connected also to the anode side of the second diode D1302. The other end of the first transmission line is connected to the cathode of the first diode D1301, and also coupled to an antenna terminal 1304 through a third capacitor element C1303. Here, in order to decrease the variety of components to be used and thereby to reduce cost, usually, diodes having the same characteristics are used for the first diode D1301 and the second diode D1302.
The operation of the high frequency circuit configured as described above will be described.
In transmitting, when a positive voltage is applied to the control terminal 1302, the first diode D1301 and the second diode D1302 are turned on. At this time, the capacitor elements C1301, C1302, and C1303 block components of direct current. A transmit signal passes through the capacitor C1301 from the transmitting terminal 1301 and is transmitted to the antenna terminal 1304 through the first diode D1301 and the capacitor element C1303. Herein, the transmission line TL1301 operates as a ¼ wavelength resonator having one end grounded, because the second diode D1302 is turned on. Therefore, the impedance of lines in the side of the antenna terminal 1304 becomes infinite and therefore no transmit signal is transmitted to the receiving side.
In receiving, no voltage is applied to the control terminal 1302, and therefore both the first diode D1301 and the second diode D1302 are in the OFF state. Thus, a receive signal is transmitted to the receiving terminal 1303 from the antenna terminal 1304 through the capacitor element C1303, the transmission line TL1301, and the capacitor element C1302.
PIN diodes are mainly used for the first diode D1301 and the second diode D1302, which are used for switching. However, generally, diodes have a tradeoff relationship that a diode of low ON resistance has a large capacitance between the cathode and anode terminals and a diode having a small inter-terminal capacitance in the OFF state has a high ON resistance.
Therefore, if importance is attached on isolation during receiving and therefore a diode having a small inter-terminal capacitance is selected to achieve high isolation, the signal path produces a large loss during transmitting because of the large ON resistance of the diode.
On the other hand, when importance is attached on insertion loss during transmitting and therefore a diode of a low ON resistance is selected, then there is a problem that the increased inter-terminal capacitance reduces isolation to result in a large loss produced by the signal path during receiving.